The invention relates to a particle accelerator with frequency correction, of the type in which the particles are accelerated under the effect of an electromagnetic wave injected into a resonant cavity.
In accelerators of this type, it is of prime importance for the frequency of the electromagnetic wave, delivered by an ultra high frequency generator for example, to correspond to a tuning frequency or to the resonance frequency of the resonant cavity, so that a maximum of electromagnetic power is transferred from the generator to the resonant cavity.
The electromagnetic wave is applied to the resonant cavity through a transmission line, generally formed by a wave guide in the case of ultra high frequencies, and a difference between the frequency of the electromagnetic wave and the tuning frequency causes reflection by the cavity of all or part of the incident power. There then exists in the wave guide, an incident wave which is propagated from the generator to the cavity and a reflected wave which is propagated from the cavity to the generator; the phase of the reflected wave varying greatly about the tuning frequency.
These phenomena are used for elaborating an error signal which is applied to the generator so as to correct any frequency deviation; the generator comprising in a way known per se means for modifying the frequency of the wave which it generates.
For this, with the frequency correction devices of the prior art, a first signal relative to the incident wave is taken from the wave guide through a first directional coupler and a second signal relative to the reflected wave is taken through a second directional coupler. These signals are applied to the opposite ends of a second transmission line, of coaxial type, and for one of these signals via an adjustable attenuator means and an adjustable phase shift means.
The superimposition of the incident wave and of the reflected wave generates a standing wave, whose maximums and minimums occupy, along the second line, positions defined by the setting of the adjustable phase shift means the ratio of a maximum to a minimum being determined by the setting of the adjustable attenuator. In addition, two detector means detect the standing wave in the second line, at judiciously spaced points placed at equal distances from a minimum of the wave, so as to obtain two measurement signals having an equal amplitude for the tuning frequency; this being obtained more especially by adjusting the phase shift means. The measurement signals are applied to a conventional type device, which works out the difference thereof and which delivers an error signal having a zero value when the frequency of the electromagnetic wave corresponds to the tuning frequency, or a positive or negative value depending on whether these frequencies are different in one direction or in the other.
The two waves, one direct and the other reflected, must be absorbed after passing through the second transmission line, which requires for each of these waves an absorber element such as an ultra high frequency insulator.
The arrangement of the means which has just been described has been known and used for a long time for controlling the frequency, more especially in particle accelerators. It may be noted, for accelerators for example, that industrial progress has considerably reduced the cost prices, but that up to present this reduction in cost has not concerned the part relative to the frequency control. This high cost is due on the one hand to the high cost of each of its elements, namely the two directional couplers, the two absorber elements, the adjustable attenuator, the variable phase shifter, the second transmission line and on the other hand because of the considerable work required for mounting, assembling and adjusting all these elements.
In fact, the succession of operations required for adjusting such a correction device is long and delicate, more especially in so far as the relative positioning between the detector means and the minimum of the standing wave is concerned.
It should be further noted that the number of these elements adversely affects the reliability of the assembly, and that a defect occuring in a simple adjustable attenuator has serious consequences in the use of a particle accelerator.